<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="UTF-8" />
    <meta http-equiv="X-UA-Compatible" content="IE=edge" />
    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
    <title>Document</title>
    <style>
      * {
        margin: 0px;
        padding: 0px;
      }
      .box {
        width: 100px;
        height: 100px;
        background-color: red;
        position: absolute;
      }
    </style>
  </head>
  <body>
    <div class="box"></div>
  </body>
</html>
<script>
  var boxEle = document.querySelector(".box");
  boxEle.onmousedown = function (e) {
    console.log("黑色的框");
    // boxEle.offsetLeft是不会改变的
    var x = e.clientX - boxEle.offsetLeft;
    var y = e.clientY - boxEle.offsetTop;
    boxEle.onmousemove = function (e) {
      console.log("移动了");
      // 在移动的时候获取 offsetleft ，offset会不停的被设置
      var xx = e.clientX;
      var sx = xx - x; // xx-(e.clientX-boxEle.offsetLeft)
      var yy = e.clientY;
      var sy = yy - y;
      // 设置left的值；
      boxEle.style.left = sx + "px";
      boxEle.style.top = sy + "px";
    };
  };
  document.onmouseup = function () {
    boxEle.onmousemove = "";
  };
</script>
